Metallic materials, in particular iron and steel, are plated with zinc or cadmium in order to protect them from corrosive environmental influences. The corrosion protection of zinc resides in the fact that it is even less precious than the base metal and therefore at first exclusively draws the corrosive attack; it acts as a sacrificial layer. The base metal of the respective zinc-plated component remains unimpaired as long as it is continuously covered with zinc, and the mechanical functionality remains preserved over longer periods of time than in the case of parts not plated with zinc. Thicker zinc layers naturally afford higher corrosion protection than thin layers inasmuch as corrosive erosion of thicker layers simply takes more time.
The corrosive attack on the zinc layer, in turn, can be greatly delayed by application of a chromation, or chromate coating, whereby corrosion of the base metal is even further postponed than by mere zinc plating. A considerably better corrosion protection is afforded by the zinc/chromate layer system is than by a mere zinc layer of identical thickness. Moreover by means of chromation the optical deterioration of a component due to environmental influences is further postponed--the corrosion products of zinc, referred to as "white rust", equally interfere with the optical appearance of a component.
The advantages of an applied chromation are so important that almost any galvanically zinc-plated surface is in addition chromate coated as well. The prior art knows four chromations named after their colorations, which are each applied by treating (immersion, spraying, rolling) a zinc-plated surface with the corresponding aqueous chromate coating solution. Moreover yellow and green chromations for aluminum are known which are produced analogously. In any case, these are variously thick layers of substantially amorphous zinc/chromium oxide (or aluminum/chromium oxide) with non-stoichiometric compositions, a certain water content, and inserted foreign ions. These are known and classified into method groups in accordance with German Industrial Standard (DIN) 50960, Part 1:
1) Colorless and Blue Chromations, Groups A and B
The blue chromate layer has a thickness of up to 80 nm, is weakly blue in its inherent color and presents a golden, reddish, bluish, greenish or yellow iridescent coloring brought about by refraction of light in accordance with layer thicknesses. Very thin chromate layers lacking almost any inherent color are referred to as colorless chromations (Group A). The chromate coating solution may in either case consist of hexavalent as well as trivalent chromates and mixtures of both, moreover conducting salts and mineral acids. There are fluoride-containing and fluoride-free variants. Application of the chromate coating solutions is performed at room temperature. The corrosion protection of unmarred blue chromations amounts to 10-40 h in the salt spray cabinet according to DIN 50021 SS until the first appearance of corrosion products. The minimum requirement for Method Groups A and B according to DIN 50961 Chapter 10 Table 3 is 8 h for drumware and 16 h for shelfware.
2) Yellow Chromations, Group C
The yellow chromate layer has a thickness of approx. 0.25-1 .mu.m, a golden yellow coloring, and frequently a strongly red-green iridescent coloring. The chromate coating solution substantially consists of hexavalent chromate, conducting salts and mineral acids dissolved in water. The yellow coloring is caused by the significant proportion (80-220 mg/m.sup.2) of hexavalent chromium which is inserted besides the trivalent chromium produced by reduction in the course of the layer formation reaction. Application of the chromate coating solutions is performed at room temperature. The corrosion protection of unmarred yellow chromations amounts to 100-200 h in the salt spray cabinet according to DIN 50021 SS until the first appearance of corrosion products. The minimum requirement for Method Group C according to DIN 50961 Chapter 10 Table 3 amounts to 72 h for drumware and 96 h for shelfware.
3) Olive Chromations, Group D
The typical olive chromate layer has a thickness of up to 1.5..mu.m and is opaquely olive green to olive brown. The chromate coating solution substantially consists of hexavalent chromate, conducting salts and mineral acids dissolved in water, in particular phosphates or phosphoric acid, and may also contain formates. Into the layer considerable amounts of chromium(VI) (300-400 mg/m.sup.2) are inserted. Application of the chromate coating solutions is performed at room temperature. The corrosion protection of unmarred olive chromations amounts to 200-400 h in the salt spray cabinet according to DIN 50021 SS until the first appearance of corrosion products. The minimum requirement for Method Group D according to DIN 50961 Chapter 10 Table 3 is 72 h for drumware and 120 h for shelfware.
4) Black Chromations, Group F
The black chromate layer is fundamentally a yellow or olive chromation having colloidal silver inserted as a pigment. The chromate coating solutions have about the same composition as yellow or olive chromations and additionally contain silver ions. With a suitable composition of the chromate coating solution on zinc alloy layers such as Zn/Fe, Zn/Ni or Zn/Co, iron, nickel or cobalt oxide will be incorporated into the chromate layer as a black pigment so that silver is not required in these cases. Into the chromate layers considerable amounts of chromium(VI) are inserted, namely between 80 and 400 mg/m.sup.2 depending on whether the basis is a yellow or olive chromation. Application of the chromate coating solutions is performed at room temperature. The corrosion protection of unmarred black chromations on zinc amounts to 50-150 h in the salt spray cabinet according to DIN 50021 SS until the first appearance of corrosion products. The minimum requirement for Method Group E according to DIN 50961 Chapter 10 Table 3 is 24 h for drumware and 48 h for shelfware. Black chromations on zinc alloys are considerably above the specified values.
5) Green Chromations for Aluminum, Group E
The green chromation on aluminum (known under the name of aluminum green) is of a matt green and not iridescent. The chromate coating solution substantially consists of hexavalent chromate, conducting salts and mineral acids dissolved in water as well as particularly phosphates and silicofluorides. Contrary to a prevailing opinion the formed chromate/phosphate layer is, as evidenced by iodised starch tests, not always 100% chromium(VI)-free. The production of aluminum green in chromate coating solutions exclusively on the basis of chromium(III) is not known.
In accordance with the prior art, thick chromate layers affording high corrosion protection&gt;100 h in the salt spray cabinet according to DIN 50021 SS or ASTM B 117-73 until the appearance of first corrosion products according to DIN 50961 (June 1987) Chapter 10, in particular Chapter 10.2.1.2, in the absence of sealing or any other particular aftertreatment (DIN 50961, Chapter 9) may only be produced by treatment with dissolved, markedly toxic chromium(VI) compounds. Accordingly the chromate layers having the named requirements to corrosion protection still retain these markedly toxic and carcinogenic chromium(VI) compounds, which are, moreover, not entirely immobilised in the layer. Chromate coating with chromium(VI) compounds is problematic with respect to workplace safety. Use of zinc-plated chromations produced with chromium(VI) compounds, such as the widespread yellow chromations e.g. on screws, constitutes a potential hazard to the population and increases the general cancer risk.
U.S. Pat. No. 4,384,902, in particular with Examples 1, 2, 4 and 5, describes conversion layers which satisfy the requirements in the salt spray test. In all of the cases, these are cerium-containing layers presenting a yellowish coloration which is accentuated by the cerium(IV) ion. The examples only contain cerium(III), and hydrogen peroxide as an oxidant, in the bath solution. In the description it is set forth that hydrogen peroxide in the acidic medium does not represent an oxidant for Ce(III), however during deposition the pH value nevertheless rises so high at the surface that a sufficient amount of Ce(IV) may be generated. The yellowish coloration achieved by this bath composition indeed appears to indicate that an oxidation has taken place--however, only an oxidation from Ce(III) to Ce(IV). Tetravalent cerium is an even more powerful oxidant than hexavalent chromium, for which reason Ce(IV) will produce from Cr(III) the Cr(VI) which is to be avoided. Cr(VI) has a very strong yellow coloration and is known as an anticorrosion agent. The layer described in U.S. Pat. No. 4,384,902 is thus not free of hexavalent chromium.